Izu Water Temperature & Suit Timing — Best Season from Historical Data

For divers in Izu, two big questions arise every year: "When should I switch from my drysuit to a wetsuit?" and "When is the best season for water temperature?" We analyzed over 12,000 days of water temperature data from 5 Izu dive sites to reveal the complete temperature timeline and optimal transition timing.

Monthly Temperatures Across 5 Izu Sites

Let's start with the monthly averages across 5 major Izu sites. Note the subtle differences between locations.

Temperature Timeline

Coldest: Feb-Mar (15-16°C)

Water temperatures bottom out at 15-16°C. Futo is coldest at 15.1°C (February). Drysuits with thick undergarments are essential. The silver lining: visibility peaks before spring plankton blooms, making winter Izu "cold but clear."

Rising: May (17-19°C)

April barely changes, but May brings Kuroshio warm water, pushing temperatures to 17.7-18.9°C. Koganezaki leads at 18.6°C. Still drysuit season, though warm days allow thick wetsuits for some.

Rapid Rise & Suit Switch: June (19-23°C)

June is the turning point — temperatures jump 2-4°C. Most shops switch to wetsuits this month. The general rule is that 20°C is the drysuit/wetsuit boundary.

• Koganezaki: Warms earliest, averaging 23.0°C in June
• Hirasawa: Sheltered bay warms quickly, reaching 21.1°C in June
• Futo: June averages 19.8°C — borderline; drysuit may be safer for those who get cold easily
• IOP: June is exactly 20.0°C, right at the boundary. A 5mm wetsuit with hooded vest is a safe choice
• Mikomoto: Open ocean with high variability. June averages 20.9°C but lows can reach 14°C depending on Kuroshio Current meanders

Peak: Aug-Sep (23-26°C)

Temperature peaks in late August through September. Hirasawa reaches 25.6°C, Koganezaki hits 26.2°C. Comfortable in 5mm wetsuits; 3mm possible on warm days. Post-typhoon temperature drops of 2-3°C are possible.

Still Warm: October (23-24°C)

Surprisingly, October maintains summer-like water temperatures. All sites stay above 23°C with stable visibility. Air cools but water stays warm — arguably the best value season.

Rapid Cooling & Autumn Switch: Nov-Dec

The autumn switch is trickier. October is still comfortable at 23-24°C, but November brings a rapid drop.

• October: 23.5-24.2°C average — comfortable in 5mm wetsuit
• November: 21.0-21.7°C average — switch to drysuit if you get cold easily
• December: 18.1-19.0°C average — drysuit for everyone

Most shops recommend switching to drysuits in mid to late November. According to Japan Meteorological Agency data, warming trends mean autumn cooling is arriving later in recent years.

Recommended Switch Timing by Preference

Best Season Summary by Site

If You Don't Own a Drysuit

Many divers don't own a drysuit. In Izu, a 5mm wetsuit with hooded vest covers about 5 months (June-October) comfortably. November at 21°C is manageable in 6.5mm for cold-tolerant divers.

However, attempting to dive in 15°C water (February-March) in a wetsuit risks hypothermia. For winter diving in Izu, invest in or rent a drysuit.